The prevalence of wireless electronic devices has placed increasing constraint on the power performance of electronic circuits. The convenience and utility of battery powered wireless devices are greatly improved through the use of low power circuits. Circuit techniques that preserve power are, therefore, increasingly important in order for these devices to keep in step with higher consumer expectations for convenience and device functionality.
Power amplifier circuits are a significant source of power consumption in wireless devices. One application example where such power consumption is particularly evident is in the transmitter circuit of a wireless device which may employ a power amplifier that boosts a signal for radio transmission. Sufficient power is needed to make an effective radio communication link between the wireless device and its receiver. However, the power required for satisfactory transmission may vary depending on factors such as; the distance between the transmitting and receiving antennas, and the presence of obstacles that may interference with the radio path. The power amplifier must thus be capable of operating over varying power levels or multiple power ranges with the least amount of power consumption so that the wireless device functionality can be extended before the battery needs replacement or recharge. There are other application examples in a wireless device where power amplifiers consume significant power and need to operate over a wide range of power such as a speaker or headphone output. Techniques that reduce the power consumption in power amplifiers are, therefore, important in meeting the consumer's requirements for wireless devices in the competitive market. However, it is difficult to optimize the electrical characteristics of a single amplifier to function over the desired range of power efficiently.
FIG. 1 illustrates an example of a prior art low power wireless system 100 including a low power wireless device 105. The low power wireless device includes a power amplifier 120 to explain one example of the use of a power amplifier using a wireless device transmitter circuit and the importance of reducing the power consumed by power amplifiers. The input of power amplifier 120 is coupled to an output of a remainder of wireless device 110. The input to remainder of wireless device 110 is coupled to the output of a linear amplifier 130. A battery 180 for supplying electrical power is coupled to remainder of wireless device 110, power amplifier 120, and linear amplifier 130. The output of power amplifier 120 and the input of linear amplifier 130 are coupled to a transmit receive switch 160. Transmit receive switch 160 is coupled to an antenna 170 which emits and receives radio communication waves 185 to and from an antenna 190 which is external of low power wireless device 105 and coupled to a wireless device infrastructure 195. Transmit receive switch 160 receives instructions (not shown) from the wireless device.
Examples of low power wireless device 105 include a cell phone, a headset, a computer mouse, or a laptop computer, to name just a few applications. Examples of wireless device infrastructure 195 include a cellular relay tower coupled to a phone network, a television coupled to a broadcast network, a personal computer, and a wireless router connected to the internet. To transmit information from the wireless device, switch 160 closes the switch between power amplifier 120 and antenna 170 while opening the receive path to linear amplifier 130. To receive information, switch 160 opens the switch between power amplifier 120 and antenna 170 while closing the switch to enable the receive path to linear amplifier 130. Radio communication between the wireless device and the wireless device infrastructure enable communication with great user convenience. That convenience ends when battery 180 runs out of charge and needs to be either replaced or recharged. Longer battery lifetime produces higher consumer satisfaction and reduces hazardous waste in the environment. Power amplifier 120 draws a significant amount of power from the battery during transmit operation of the wireless device so it is important to optimize the amplifier's power efficiency.
Many communication systems need to accommodate a wide range of output transmission power levels. For example, good transmission conditions are obtained when radio communication waves 185 have a comparatively short distance to travel, antennas 170 and 190 are in a more direct line of sight, and there are few obstacles in the transmission area that can cause interference. Under these good transmission conditions, there is no need to operate power amplifier 120 with as much power output as in bad transmission conditions. During good transmission conditions, it is desirable to reduce the amplifier's power output to extend the battery lifetime. Thus, power amplifier 120 may be required to operate over a wide range of power output. However, it is generally true that a single conventional power amplifier such as amplifier 120 has a certain range of output power over which its efficiency is high and operation at other power ranges results in worse power efficiency. For example, a conventional single power amplifier that is efficient at high powers will not be as power efficient at low powers.
Each foregoing example is provided by way of explanation of the contextual background of the invention, not limitation of the scope of invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the foregoing examples without departing from the spirit and scope thereof. For instance, the low power wireless device may have coupled to it a plurality of antennas. Also, low power amplifiers that operate over a wide range of power are useful in non-wireless applications such as hearing aids, ear phones, portable instrumentation, and other electronic applications.
One solution to the challenge of optimizing amplifier power efficiency over a wide range of power is to use multiple amplifiers. Several circuits in a cellular communication system using multiple amplifiers are disclosed in U.S. Pat. No. 5,872,481, entitled “Efficient Parallel-Stage Power Amplifier” including; a circuit utilizing an output network connected to each amplifier, and a circuit with two amplifiers that are simultaneously biased in an active state with a switch that shunts one amplifier output to the antenna while shunting the other amplifier output to ground through a load device.
Thus it is desirable to optimize power amplifier efficiency over a wide range of power in order to extend the battery lifetime and functionality of wireless devices.